Cross-linked thermoplastic polyurethane/polyurea and method of making same

ABSTRACT

A cross-linked thermoplastic polyurea is formed by heating a mixture containing a thermoplastic urethane base material, a monomeric and/or polymeric di-isocyanate comprising between 1 to 10% of the total weight of the mixture, and a diamine comprising between 1 to 10% of the total weight of the mixture. A thermoplastic polyurethane may be formed by substituting hydroquinone for the diamine. The mixture is heated to a temperature within the range of 250° F. to 550° F. The heated mixture, which is flowable, is then injected into at least one injection molding device. The mixture is then cured at a temperature between 150° F. to 250° F. for a period of time between 2 and 36 hours. The cross-linked thermoplastic polyurethane/polyurea retains the excellent flowability characteristics of a thermoplastic urethane while the same time exhibits good abrasion, tensile strength, rebound, and compression set characteristics which are similar to those found in thermoset urethanes.

CROSS REFERENCES TO RELATED APPLICATIONS

The Present Application is a divisional application of U.S. patentapplication Ser. No. 10/992,907, filed on Nov. 18, 2004, issued as U.S.Pat. No. 7,417,094 B2 Aug. 26, 2008.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention generally relates to thermoplasticpolyurethanes and thermoplastic polyureas having properties similar tothose of castable or cross-linked polyurethanes or polyureas. The fieldof the invention also includes methods of making the same.

2. Description of the Related Art

There currently are a number of commercialized products made frompolyurethanes and polyureas. Typically, these products made from eitherthermoplastic polyurethanes (or polyureas) or thermoset polyurethanes(or polyureas). Thermoplastic polyurethanes generally have linearmolecular structures and are able to flow freely at elevatedtemperatures. For this reason, thermoplastic polyurethanes are preferredfor products which are produced by injection molding or other extrusiontechniques, where flowability of the reactants are of paramountimportance. Unfortunately, thermoplastic polyurethanes typically exhibitpoor performance characteristics with respect to abrasion, tensilestrength, rebound, and compression set compared to castablepolyurethanes.

In contrast to current thermoplastic polyurethanes, thermosetpolyurethanes have particularly good characteristics with respect toabrasion, tensile strength, rebound, and compression set. Thermosetpolyurethanes generally have a network structure that incorporatesirreversible chemical cross-linking. The downside of thermosetpolyurethanes is that the irreversible chemical cross-linking reactionmakes it unsuitable for use in injection molding and extrusionapplications. Typically, thermoset polyurethanes are formed using acasting process. Unfortunately, casting processes require costlyequipment and usually involve a large number of processing steps.Casting is thus a less efficient and more expensive method of producingpolyurethane-based and polyurea-based products as compared to injectionmolding and extrusion systems.

In a typical process for making a thermoset (i.e., castable)polyurethane, a di-isocyanate component is first pre-polymerized with apolyol having either a polyester or polyether backbone. The remainingdi-isocyanate of the pre-polymer is reacted with a chain extender or across-linking agent or a blend of cross-linking agents. Catalysts areadded to control the reaction rate. If the cross-linking agent has adihydroxy functional component, a polyurethane will be formed. If thecross-linking agent has diamine functionality, a polyurea is formed.

With respect to thermoplastic polyurethanes, a diol or polyol is reactedwith an isocyanate. This reaction typically takes place in largecommercial reactors. As stated above, thermoplastic polyurethanes, whilenot cross-linked, are usable in injection molding and other extrusionmethods. Because of the lack of cross-linking, these materials haveabrasion, tensile, and compression set properties that are not as goodas thermoset polyurethane or polyurea systems.

There thus is a need for a thermoplastic polyurethane or polyureamaterial which exhibits good abrasion, tensile strength, rebound, andcompression set characteristics which are similar to those found inthermoset urethanes. Such a material could be produced usingconventional injection molding and/or extrusion techniques, therebyreducing the cost of manufacture for the material.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the invention, a cross-linked thermoplastic polyureaincludes a mixture of thermoplastic urethane base material, a monomericdi-isocyanate comprising between 1 to 10% of the mixture on a totalweight basis, and a diamine comprising between 1 to 10% of the mixtureon a total weight basis.

In another aspect of the invention, a method of making a cross-linkedthermoplastic polyurea is provided. The method includes the steps ofproviding a mixture containing a thermoplastic urethane base material, amonomeric and/or a polymeric di-isocyanate comprising between 1 to 10%of the total weight of the mixture, and a diamine comprising between 1to 10% of the total weight of the mixture. The mixture is then heated toa temperature within the range of 250° F. to 550° F. The heated mixtureis then injected into at least one injection molding device.Post-injection, the material is cured at a temperature between 150° F.to 250° F. for a period of time between 2 and 36 hours.

In another aspect of the invention, a cross-linked thermoplasticpolyurethane includes a mixture of thermoplastic urethane base material,a monomeric di-isocyanate comprising between 1 to 10% of the mixture ona total weight basis, and hydroquinone comprising between 1 to 10% ofthe mixture on a total weight basis.

It is an object of the invention to provide a thermoplastic polyurethaneor polyurea material which exhibits good abrasion, tensile strength,rebound, and compression set characteristics which are similar to thosefound in thermoset urethanes. It is a further object of the invention toprovide a method of producing such a material using conventionalinjection molding and/or extrusion techniques. Additional objects of theinvention are disclosed below.

Having briefly described the present invention, the above and furtherobjects, features and advantages thereof will be recognized by thoseskilled in the pertinent art from the following detailed description ofthe invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 schematically illustrates an injection molding device which canbe used to produce products made from cross-linked thermoplasticpolyurethanes/polyureas according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A cross-linked thermoplastic polyurethane or polyurea is formed using apolyester or polyether backbone material. The polyester or polyetherbase material can include, for example, commercial grade thermoplasticurethanes. For instance, the thermoplastic urethane base material mayinclude TEXIN 985, an aromatic polyether-based thermoplasticpolyurethane having a Shore hardness of about 85. TEXIN 985 can beobtained from Bayer Corporation, 100 Bayer Road, Pittsburgh, Pa. 15205.As another example, the thermoplastic urethane base material may includeNOVEON ST80A, which is available from Noveon, Inc., 9911 BrecksvilleRoad, Cleveland, Ohio 44141-3247.

The thermoplastic urethane base material is preferably dried prior toadding the additional components described in detail below. This can beaccomplished, for example, by heating thermoplastic base material to atemperature between about 100° F. to 200° F. in a separate container.

A monomeric di-isocyanate (MDI) is added to the dried thermoplasticurethane base material. Preferably, the MDI used in the process is asolid at room temperature. In one preferred aspect of the invention, theMDI is 4,4′ diphenylmethane di-isocyanate. This can be commerciallyobtained from Bayer Corporation under the trade name, MONDUR M (CAS No.101-68-8). Preferably, the flaked or fused form of MONDUR M is used inconnection with the process described herein. The MDI is preferablystored at about −20° C. By adding MDI to the thermoplastic urethane basematerial, isocyanate functionality is added to the system. OtherDi-isocyanate materials which are solid at room temperature and may beused in accordance with the invention include: Toluene Di-isocyanates(TDI), Toluene ortho Di-isocyanates (TODI), Naphthalene Di-isocyanates(NDI), Hydrogenated Methylene Di-isocyantaes (H12MDI), Iso PhoroneDi-isocyanates (IPDI), Hexamethylene Di-isocyantes (HDI). Theseisocyanate-based compounds can be made in solid crystalline formsuitable for dry blending. These isocyanates can also be added in theliquid and semi-liquid form.

Preferably, MDI comprises between 1% to 10% of the total weight of themixture forming the cross-linked thermoplastic polyurethane/polyurea.Even more preferably, MDI comprises between 1% to 2% of the total weightof the mixture forming the cross-linked thermoplasticpolyurethane/polyurea. MDI materials other than those specificallyidentified above may also be used in accordance with the invention,provided they exist as a solid at room temperature.

The cross-linked thermoplastic urethane also includes a diamine which isused to cross-link the liquid thermoset urethane. One preferred diamineis 4,4′ methylene-bis-(3-chloro-2,6-diethylaniline), availablecommercially as LONZACURE M-CDEA (CAS No. 106246-33-7). Another diaminewhich can be employed with the present invention is 4,4′Methylene-bis-(2,6-diethylaniline), available commercially as LONZACUREM-DEA (CAS No. 13680-35-8). Both diamines have melting points atapproximately 90° C. Preferably, the diamine is added in solid form anddry blended with the MDI and thermoplastic urethane base material.Alternative cross-linking agents and other solid or crystalline Diamineswhich may be used in the present invention include: MOCA(4,4′-Methylenebis-(O-Chloroaniline)), MDA (Methylene Dianiline), aswell as any other methylene bis aniline like LONZACURE M-CDEA describedabove. Any other diamine-based compounds can be made in solidcrystalline form suitable for dry blending can also be used. Thediamines above can also be added in the liquid or semi-liquid form.

Preferably, diamine comprises between 1% to 10% of the total weight ofthe mixture forming the cross-linked thermoplastic polyurea. Even morepreferably, diamine comprises between 1% to 2% of the total weight ofthe mixture forming the cross-linked thermoplastic polyurea. Diaminesother than those specifically identified above may also be used inaccordance with the invention, provided they exist as a solid at atemperature within the range of 50° F. to 150° F.

In an alternative embodiment, hydroquinone (HQEE) replaces the diamineconstituent and is added to the mixture of MDI and thermoplasticurethane. As with the prior embodiment, HQEE is added to the mixture insolid form and dry blended with the MDI and thermoplastic urethane basematerial. In yet another alternative embodiment, HQEE is added inconjunction with a diamine.

The mixture of thermoplastic urethane base material, MDI, and diamine(and/or HQEE) is then mixed and heated to a temperature within the rangeof 250° F. to 550° F. The solid thermoplastic urethane base material,MDI, and diamine (and/or HDEE) melt and partially cross-link.Preferably, the partially cross-linked thermoplasticpolyurethane/polyurea is post-cured by heating the same to a temperaturewithin the range of 150° F. to 250° F. for a period of time rangingbetween 2 and 36 hours.

FIG. 1 illustrates an injection molding device 10 capable of producingcross-linked thermoplastic polyurethanes/polyureas in accordance withthe present invention. The injection molding device 10 includes a hopper12 for loading the various components (i.e., thermoplastic urethane basematerial, MDI, and diamine/HQEE). As shown in FIG. 1, the hopper 12 ispreferably partitioned into a plurality of separate bins 12(a), 12(b),12(c), 12(d) for loading the various components in the mixture.

A weight chamber 14 is positioned underneath the hopper 12 for measuringthe weight of the materials being added via the various bins (e.g.,12(a), 12(b), 12(c), 12(d)). A mixing chamber 16 is positioned beneaththe weight chamber 14 and includes a rotating mixer 18 therein for drymixing the constituents for the cross-linked thermoplasticurethane/urea.

The mixing chamber 16 communicates with screw 20 via a port 16(a). Thescrew 20 is disposed inside a mixing barrel 22. The screw 20 bothrotates and reciprocates within the mixing barrel as is shown by arrowsA and B, respectively. A plurality of heating bands 24 are disposedcircumferentially around the mixing barrel 22 to heat the mixture as ittravels along the screw 20. The plurality of heating bands 24 create aplurality of heating zones along the length of the screw 20. Preferably,the heating bands 24 can each be independently controlled to createdifferential temperatures along the length of the screw 20. For example,a temperature gradient may be established along the length of the screw20 during operation of the device 10. The distal end of the mixingbarrel 22 terminates into an injection chamber 26 and injection nozzle28. The injection nozzle 28 is disposed inside a stationary platen 30and communicates with a sprue bushing and runner 32 in one of twoseparable mold plates 34(a), 34(b). Mold plate 34(a) is affixed orstationary with respect to stationary platen 30. In contrast, mold plate34(b) is affixed to moveable platen 36. Moveable platen 36 is moveablein the direction of arrow C shown in FIG. 1.

The mold plates 34(a), 34(b) include one or more cavities (not shown)having a pre-formed shape. The injection molding device 10 may be usedto form any number of products including, for example, skateboardwheels, in-line skate wheels, roller coaster wheels, caster wheels,castable urethane. Products in the automotive industry such as seals,O-rings, gaskets, bushings, CV-joint cover, and tires may also be madeusing the methods described herein. For agricultural applications, themethods can be used in silo liners, plow parts, pipe, and pipe liners.The invention also has utility in mining applications, where the methodsand processes described herein can be used to produce mining screens,material moving buckets, pump parts and liners, pulleys, and bumpers.The materials and methods can also be used in footwear applications suchas, for example, shoe soles and the like. The invention can also be usedin general purpose applications such as press pads, abrasion-resistantsilo or hopper liner sheets, gears, hammers, metal forming parts, etc.

The methods and materials described herein are applicable to anycast-based, injection mold-based, or extrusion-based process whichrequire a thermoplastic urethane with good abrasion, tensile strength,rebound, and compression set characteristics which are similar to thosefound in thermoset urethanes.

With reference to FIG. 1, the various mixture constituents are added tothe hopper bins 12(a)-12(d). The constituents are weighed in the weightchamber 14 and mixed inside the mixing chamber 16. The mixed materialthen passes to the mixing barrel 22. Preferably, the temperature of thebarrel zones (those portions of the mixing barrel 22 adjacent to theheating bands 24) are kept within the range of 250° F. to 550° F. Theinjection nozzle 28 is also preferably kept at a temperature within therange of 250° F. to 550° F. While the constituents pass through thescrew 20, they melt and partially cross-link.

After the melted mixture is injected into the mold cavities (not shown),the moveable mold plate 34(b) is moved in the direction of arrow C toopen the cavity. The product is then removed and cured by heating thesame at a temperature within the range of 160° F. to 230° F. for 6 to 18hours.

The injection molding cycle time using the above-described cross-linkedpolyurethane/polyurea is very fast when compared to the conventionalcycle time of a castable thermoset method. The equipment and ancillarytooling needed to produce products made by injection molding (i.e.,FIG. 1) is much less when compared to casting methods. For example, afour-cavity injection molding tool on a 200-ton machine could produce,with a single operator, as many parts as six operators using over 60casting tools with a single cavity each. Enormous efficiencies aregained in injection molding systems as compared to current casting-basedsystems.

The cross-linked thermoplastic polyurethane/polyurea described herein isable to be efficiently (and cheaply) produced using injection moldingtechnology without sacrificing performance characteristics.

The following are experimental test results of various cross-linkedthermoplastic urethanes/ureas in accordance with the present invention.

Experiment 1

In this experiment, TEXIN 985 was used as the thermoplastic urethanebase material. Different amounts of MDI in the form of MONDUR M wereadded to the mixture (ranging from 1% to 2% by weight of the totalmixture). Diamine in the form of LONZACURE M-CDEA was also added to themixture (in amounts ranging from 1% to 2% by weight of the totalmixture). The barrel zone temperature was set at 380° F. (ejectionnozzle). The remaining zones where set to 390° F. The material waspost-cured at 200° F. for 12 hours.

Table 1 listed below illustrates the performance characteristics of thecontrol (100% thermoplastic urethane base material) as well as threevarious weight percentages of MONDUR M and LONZACURE M-CDEA.

TABLE 1 A B C D Bayer Testing Reference Numbers NB 893029A NB 893029B NB893029C NB 893029D Percent of Base and Additives Bayer Stated Base: 100%Base: 100% Base: 100% Present Mechanical Base: 100% Add-1: 1% Add-1: 1%Add-1: 2% Improvement Over Properties (Control) Add-2: 1% Add-2: 2%Add-2: 2% Stated Properties Hardness, Shore A 85 78 80 78 81 −4.71%Taber Abrasion, mg loss 30 13 13 8.0 3.5 88.33% H-18 Wheel, 1000 g Load,1000 Cycles Bayshore Resilience, % 45 47.8 44.8 42.6 41.0 −8.89% TensileStrength, psi 5,500 4,084 4,425 4,623 4,594 −16.47% Tensile Stress @100% Elongation, psi 800 770 794 845 889 11.13% Tensile Stress @ 300%Elongation, psi 1200 1,258 1,375 1,565 1,675 39.58% Ultimate Elongation,% 500 661 578 484 478 −4.40% Compression Set, % 22 Hours @ 23° C. 1610.8 3.7 2.0 2.2 86.25% 22 Hours @ 70° C. 40 24.6 19.2 19.4 13.7 65.75%Base: TEXIN 985 Add-1: MONDUR M Add-2: LONZACURE M-CDEA

Experiment 2

In this experiment, TEXIN 985 was again used as the thermoplasticurethane base material. Different amounts of MDI in the form of MONDUR Mwere added to the mixture (ranging from 1% to 2% by weight of the totalmixture). Diamine in the form of either LONZACURE M-CDEA or LONZACUREM-DEA was also added to the mixture in amounts ranging from 1% to 2% byweight of the total mixture. HQEE was added in several runs ranging from1% to 2% by weight of the total mixture (runs C, E, and F). Table 2below illustrates the results of this experiment.

TABLE 2 A B C D E F Bayer Testing Reference Numbers Base BayerThermoplastic Urethane NB 893044A NB 893044B NB 893044C NB 893044D NB893044E NB 893044F Texin 985 Texin 985 Texin 985 Texin 985 Texin 985Texin 985 Percent of Base and Additives Base: 100% Base: 100% Base: 100%Base: 100% Base: 100% Bayer Stated Add-1: 2% Add-1: 2% Add-1: 2% Add-1:2% Add-1: 2% Mechanical Add-2: 2% Add-2: 0% Add-2: 0% Add-2: 1% Add-2:0% Properties Base: 100% Add-3: 0% Add-3: 0% Add-3: 2% Add-3: 0% Add-3:1% for Texin 985 (Control) Add-4: 0% Add-4: 2% Add-4: 0% Add-4: 1%Add-4: 1% Shore Hardness, A Scale 85 81 83 85 85 84 84 Taber Abrasion,mg loss 30 30.8 16.3 36.0 18.0 26.8 38.8 H-18 Wheel, 1000 g Load, 1000Cycles Bayshore Resilience, % 45 51.0 49.0 46.6 50.2 49.0 46.8 TensileStrength, psi 5,500 3,277 3,833 4,021 3,507 3,508 3,545 Tensile Stress @100% Elongation, psi 800 788 831 842 842 834 835 Tensile Stress @ 300%Elongation, psi 1200 1,299 1,466 1,580 1,404 1,558 1,469 UltimateElongation, % 500 677 563 552 635 558 602 Compression Set, % 22 Hours @23° C. 16 15.2 13.6 15.7 14.6 13.8 10.8 22 Hours @ 70° C. 40 35.5 38.533.2 38.1 37.6 50.8 Base: Texin 985 Add-1: Mondur M, Add-2: LonzacureMCDEA, Add-3 Lonzacure MDEA, Add-4 HQEE.

In this particular experiment, during mixing of the additives to thethermoplastic urethane base material, a static charge was present in theair and portions of the additives did not mix into the thermoplasticurethane base material. Consequently, the improvements in performancecharacteristics were not as dramatic as those in experiment 1. However,by comparing the Taber Abrasion results for those runs with at least 2%diamine, a reduction of at least approximately 50% was seen in TaberAbrasion values.

Experiment 3

In this experiment, NOVEON ST80A was used as the thermoplastic urethanebase material. MDI in the form of MONDUR M was added to the mixture (2%by weight of the total mixture). Diamine in the form of either LONZACUREM-CDEA or LONZACURE M-DEA was also added to the mixture in amountsranging from 1% to 2% by weight of the total mixture. HQEE was added inseveral runs ranging from 1% to 2% by weight of the total mixture (runsI, K, and L). Table 3 below illustrates the results of experiment 3.

TABLE 3 G H I J K L Bayer Testing Reference Numbers Base NoveonThermoplastic Urethane NB 893044G NB 893044H NB 893044I NB 893044J NB893044K NB 893044L ST80A ST80A ST80A ST80A ST80A ST80A Percent of Baseand Additives Base: 100% Base: 100% Base: 100% Base: 100% Base: 100%Noveon Stated Add-1: 2% Add-1: 2% Add-1: 2% Add-1: 2% Add-1: 2%Mechanical Add-2: 2% Add-2: 0% Add-2: 0% Add-2: 1% Add-2: 0% PropertiesBase: 100% Add-3: 0% Add-3: 0% Add-3: 2% Add-3: 0% Add-3: 1% for ST80A(Control) Add-4: 0% Add-4: 2% Add-4: 0% Add-4: 1% Add-4: 1% ShoreHardness, A Scale 77 80 81 80 80 79 Taber Abrasion, mg loss 27.8 13.018.8 12.8 37.3 20.3 H-18 Wheel, 1000 g Load, 1000 Cycles BayshoreResilience, % 74.2 74.0 71.4 71.0 71.4 68.6 Tensile Strength, psi 2,4503,580 1,541 2,467 2,423 1,552 Tensile Stress @ 100% Elongation, psi 538630 571 606 601 568 Tensile Stress @ 300% Elongation, psi 1,039 1,2221,067 1,157 1,136 1,033 Ultimate Elongation, % 760 662 612 698 708 685Compression Set, % 22 Hours @ 23° C. 9.5 8.9 11.8 10.0 11.8 14.0 22Hours @ 70° C. 24.8 20.8 30.2 37.0 24.6 32.6 Base: NOVEON ST80A Add-1:MONDUR M Add-2: LONZACURE M-CDEA Add-3: LONZACURE M-DEA Add-4: HQEE

In experiment 3, as with experiment 2, during mixing of the additives tothe thermoplastic urethane base material, a static charge was present inthe air and portions of the additives did not mix into the thermoplasticurethane base material. Thus, the improvement in performance parameterswas not as significant as those seen in experiment 1. Nonetheless, asseen in columns H, I, and J above in Table 3, Taber Abrasion was reducedby approximately half as compared to the control.

From the foregoing it is believed that those skilled in the pertinentart will recognize the meritorious advancement of this invention andwill readily understand that while the present invention has beendescribed in association with a preferred embodiment thereof, and otherembodiments illustrated in the accompanying drawings, numerous changes,modifications and substitutions of equivalents may be made thereinwithout departing from the spirit and scope of this invention which isintended to be unlimited by the foregoing except as may appear in thefollowing appended claims. Therefore, the embodiments of the inventionin which an exclusive property or privilege is claimed are defined inthe following appended claims.

1. A method of making a cross-linked thermoplastic polyurea comprisingthe steps of: heating a dry mixture to a temperature within the range of250° F. to 550 ° F. within a mixing barrel to created a heated partiallycross-linked mixture, the dry mixture comprising a thermoplasticurethane base material, a di-isocyanate comprising between 1 to 10% ofthe total weight of the mixture, the di-isocyanate being one ofmonomeric or polymeric, and a diamine comprising between 1 to 10% of thetotal weight of the mixture, the diamine selected from the groupconsisting of 4, 4′ methylene-bis-(3-chloro-2, 6-diethylaniline) and 4,4′ methylene-bis-(2, 6-diethylaniline); injecting the heated partiallycross-linked mixture into at least one injection mold cavity; and curingthe injected material at a temperature between 150° F. to 250° F. for aperiod of time between 2 and 36 hours.
 2. The method of claim 1 whereinthe di-isocynate comprises 4, 4′ diphenylmethane di-isocyanate.